Required practical 10: direction of growth of a root or shoot

Introduction to Required Practical 10

In this practical, we explore how plants respond to their environment. Unlike animals, plants can't walk away if they are uncomfortable, so they adapt by changing their direction of growth. This movement in response to a directional stimulus is called a tropism. In Unit 4 (Section 3.4.5), you learned the theory behind plant growth substances; here, we put that theory into practice by investigating how variables like light or gravity affect the growth of roots and shoots.

The Core Science: Plant Growth Substances

Before starting the practical, it is helpful to remember the role of Indoleacetic acid (IAA). This is a type of auxin that controls cell elongation. It works differently depending on where it is in the plant:

1. In Shoots: A high concentration of IAA promotes cell elongation. This causes the shoot to bend toward a stimulus like light.
2. In Roots: A high concentration of IAA inhibits cell elongation. This helps the root grow downward into the soil.

Setting Up the Investigation

The syllabus requires you to investigate the effect of a suitable variable on growth direction. Most commonly, this involves Phototropism (response to light) or Gravitropism (response to gravity).

Example: Investigating Phototropism in Shoots

1. Prepare the Organism: Use fast-growing seedlings, such as cress, mustard, or mung beans. Ensure they are all approximately the same age and height at the start.
2. Set Up Conditions: Divide the seedlings into groups:
- Group A (Control): All-around light (e.g., on a windowsill or under a lamp).
- Group B (Unidirectional light): Place inside a box with a small hole on one side so light only enters from one direction.
- Group C (Darkness): Place inside a completely sealed box with no light.
3. Timeframe: Leave the seedlings for 24–48 hours to allow enough growth for a measurable response.

Example: Investigating Gravitropism in Roots

To see how roots respond to gravity, you can use petri dishes lined with moist cotton wool.
- Pin germinating seeds to the cotton wool so the roots are horizontal.
- Rotate some dishes so the seeds stay horizontal (experimental group) and keep others vertical (control).
- Use a clinostat (a device that rotates slowly) for the control group to effectively "cancel out" the constant pull of gravity from one direction.

Taking Readings and Recording Data

To turn this into a scientific analysis, you need to collect quantitative (numerical) data. You might measure:

1. The Angle of Curvature: Use a protractor to measure the angle at which the shoot or root has bent from its original vertical or horizontal position.
2. Length of Growth: Use a ruler or a piece of string (to follow the curve) to see how much the tissues have elongated.
3. Mean Values: Always use a large sample size (e.g., 10 seedlings per condition) to calculate a mean and reduce the effect of random error.

Planning and Controlling Variables

For your results to be valid, you must control all other variables that could affect growth. Don't worry if this seems like a lot to remember; just think about what a plant needs to survive!

Variables to control:
- Temperature: Use a thermometer to ensure all groups are kept at the same temperature, as enzymes involved in growth are temperature-dependent.
- Moisture/Water: Ensure each group receives the same volume of water.
- Species: Use the same type of seed for every group.
- Time: Measure all seedlings after the exact same duration.

Evaluating the Procedure

In your exam, you might be asked to evaluate why an experiment didn't work as expected or how to improve it. Here are some common points to consider:

1. Difficulty in Measuring: Measuring the angle of a tiny, curved cress seedling can be difficult and subjective. Using digital photography and image analysis software can increase accuracy.
2. Light Leakage: In phototropism experiments, ensure the "dark" boxes are truly light-proof by using black tape around the edges.
3. Statistical Significance: Since we are comparing means between different groups (e.g., light vs. dark), a t-test (though not specifically named in this chapter's math requirements, it is a standard tool for comparing means) could be used to see if the difference is significant.

Quick Review: Key Takeaways

- Tropism: A growth response to a directional stimulus.
- Variable: You can test light (phototropism) or gravity (gravitropism).
- Mechanism: IAA accumulates on the shaded or lower side. In shoots, it causes elongation (bending toward light/upward). In roots, it prevents elongation (bending downward).
- Measurement: Use angles (\( ^\circ \)) and mean values for better data quality.

Common Mistakes to Avoid

1. Confusing Roots and Shoots: Remember, IAA promotes growth in shoots but inhibits it in roots. This is a very common exam trap!
2. Forgetting the Control: Always mention a control group (like seedlings in total darkness or on a clinostat) to show that the stimulus you are testing is actually the cause of the movement.
3. Measuring "Movement": Avoid saying plants "move" toward light like an animal would. Always use the phrase "directional growth" or "differential elongation".

For more information on the theory of how IAA works at the cellular level, refer to the notes on Unit 4, Section 3.4.5: Control systems in plants.